Interpretive Summary: A study of the time required for initiating an active defense response to the pathogen Magnaporthe grisea will allow the development of new approach to produce durable blast resistant rice cultivars. After inoculation with M. grisea, phenotypic responses, autofluorescence, and expression of four defense-related genes at different time intervals of rice and rice lesion mimic mutant were investigated. Lesions, either caused by disease infection or by rapid cell death (resulting from the active defense response of the plant), were not visible 24 h after inoculation. A lesion mimic mutant of rice cultivar Katy was shown to be resistant to some virulent Magnaporthe grisea isolates, and the lesion mimic phenotype was rapidly induced by virulent M. grisea isolates or by avirulent isolates at high levels of inoculum. Using a localized inoculation method, lesion mimic phenotype was positionally induced by M. grisea. Autofluorescence (a sign of an active defense response) was visible under ultraviolet light 24 h after localized inoculation in the incompatible interaction whereas, autofluorescence was not evident in the compatible interaction. Autofluorescence was also observed in a lesion mimic plant 20 h after inoculation with both virulent and avirulent isolates, thus confirming that rapid cell death is a mechanism to restrict pathogen invasion. Rapid transcript accumulation of defense and pathogenesis related genes, phenylalanine ammonia lyase and ß-glucanase, was observed beginning at 6 h and was obvious at 16 and 24 h after inoculation. Rapid transcript accumulation of PR-1 and chitinase occurred 24 h after inoculation in the resistance reaction. Accumulation of these transcripts was delayed in the susceptible reaction. These results indicate that host active defense responses can occur 24 h after pathogen inoculation in the rice blast system and that lesion mimic mutant of Katy exhibited an enhanced resistance to the rice blast pathogen.

Technical Abstract:
In the rice blast system, the timing of active defense responses by the host is unclear and was investigated by observing phenotypic responses, autofluorescence, and expression of four defense-related genes at different time intervals. Lesions, either caused by disease infection or by rapid cell death resulting from the active defense response of the plant, were not visible 24 h after inoculation. A lesion mimic mutant of rice cultivar Katy was shown to be resistant to some virulent Magnaporthe grisea isolates, and the lesion mimic phenotype was rapidly induced by virulent M. grisea isolates or by avirulent isolates at high levels of inoculum. Using a localized inoculation method, lesion mimic phenotype was positionally induced by M. grisea. Autofluorescence (a sign of an active defense response) was visible under ultraviolet light 24 h after localized inoculation in the incompatible interaction whereas, autofluorescence was not evident in the compatible interaction. Autofluorescence was also observed in a lesion mimic plant 20 h after inoculation with both virulent and avirulent isolates, thus confirming that rapid cell death is a mechanism to restrict pathogen invasion. Rapid transcript accumulation of defense and pathogenesis related genes, phenylalanine ammonia lyase and ß-glucanase, was observed beginning at 6 h and was obvious at 16 and 24 h after inoculation. Rapid transcript accumulation of PR-1 and chitinase occurred 24 h after inoculation in the resistance reaction. Accumulation of these transcripts was delayed in the susceptible reaction. These results indicate that host active defense responses can occur 24 h after pathogen inoculation in the rice blast system and that lesion mimic mutant of Katy exhibits enhanced resistance to the rice blast pathogen.